This relates to imaging devices, and more particularly, to imaging devices with pixels having stacked photodiodes.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. The image pixels generate image signals in multiple color channels. Circuitry is commonly coupled to each pixel column for reading out image signals from the image pixels.
Conventional imaging systems employ a single image sensor in which the visible light spectrum is sampled by red, green, and blue (RGB) image pixels arranged in a Bayer mosaic pattern. The Bayer mosaic pattern consists of a repeating cell of two-by-two image pixels, with two green pixels diagonally opposite one another, and the other corners being red and blue. In such an arrangement each image pixel includes a color filter that allows red, green, or blue light to pass. Each image pixel includes a single photodiode that receives the red, green, or blue light passed through the filter and generates an electrical signal indicative of the amount (i.e., intensity) of the light received through the filter.
In another arrangement, image sensor pixels are configured such that a single image pixel samples red, green, and blue light. Image pixels that sample red, green, and blue light include three photodiodes stacked one on top of another. Differences in the absorption of each of the red, green, and blue wavelengths as light passes through the silicon wafer in which the photodiodes are formed causes each of the three photodiodes to receive primarily one of the colors of light. However, the spectral separation of red, green, and blue light based on absorption depth in the silicon wafer does not sufficiently separate the light to provide isolated red, green, and blue signals that do not include at least some signal representative of light of another color. For example, a blue photodiode may be sensitive to red light that was absorbed prior to reaching the red photodiode, resulting in a blue image signal that is also representative of at least some red light. Performing color correction operations to remove these undesired responses to light of the other two colors causes undesirable amplification of system noise.
Some image pixels include two stacked photodiodes at different depths in the silicon wafer. Each photodiode is configured to absorb a different color of light. The image pixel also has a color filter formed above the stacked photodiodes that absorbs light that neither of the photodiodes is configured to absorb. The two stacked photodiodes may be sensitive to two colors of light having sufficient spectral separation in silicon such that each photodiode absorbs less of the light that the other photodiode is intended to receive (compared to, for example, a pixel having three stacked photodiodes). The color filter may block light having a wavelength that is in between the desired sensitivities of the two photodiodes (i.e., light that neither photodiode is configured to detect). The signals generated by each respective photodiode therefore provide a signal that more accurately represents the respective color of light that the photodiode is intended to receive. While this arrangement offers improved spectral separation, the shallow photodiode still absorbs some of the light that the deeper photodiode is configured to detect, and the deep photodiode may receive some light other than the wavelength it is configured to receive. Color correction operations used to correct for these undesired components in the signals introduces undesirable noise.
It would therefore be desirable to be able to provide imaging devices that generate image signals having reduced noise.